The present invention relates to an optical fiber type part for optical systems which makes use of an optical part such as an optical isolator or an optical circulator and which can be used in optical communication and optical measurement systems.
If light rays emitted from a light source are transmitted through an optical system, a part of the light rays are reflected by the end faces of the optical system and the reflected light rays come back to the light source. In transmitting signals through an optical fiber, for instance, a light beam emitted from a laser is projected on the end face of the optical fiber through lenses and the majority thereof is transmitted through the optical fiber as the transmitted light, but a part thereof undergoes surface reflection on the surfaces of the lenses and/or the end face of the fiber and come back to the laser. This leads to disturbance of the laser oscillation and becomes a cause of generating noises. An optical isolator or an optical circulator is in general used for preventing or eliminating the generation of such noises. An optical fiber type part for optical systems generally comprises a combination of an optical element such as an optical isolator or an optical circulator with an optical system.
If the refractive index of the usual glass is assumed to be 1.5, 4% of the incident light rays per end face of a glass part is reflected due to the difference between the refractive indices of the glass and the air (refractive index: 1.0) and this corresponds to a loss of the transmittance equal to 0.177 dB per end face. An optical isolator or an optical circulator usually has two end faces and accordingly, the loss is accordingly equal to 0.354 dB in all. In general, when spatially transmitting light rays through an optical fiber system such as an optical isolator or an optical circulator, the loss observed during such transmission comprises the coupling loss due to the lens system ranging from 0.3 to 0.4 dB, the transmission loss within the optical isolator or optical circulator ranging from 0.1 to 0.2 dB and the loss due to the reflection by the end faces of the optical fiber ranges from 0.5 to 0.7 dB. The generation of a transmission loss on the order of not less than 0.35 dB greatly affects the light transmission and therefore, a part provided with an anti-reflection film must be fitted to the end surfaces of optical fibers to thus eliminate such a transmission loss.
As shown in FIG. 2, a conventional optical fiber type part 10 for optical systems comprises an optical element 9 such as an optical isolator or an optical circulator and a first optical fiber 1 and a second optical fiber 2 arranged on the light-incident side and the light-outgoing side of the optical element 9 through a first lens 11 and a second lens 12 respectively. The end faces of the first and second optical fibers 1 and 2 which are opposed to one another are subjected to abrasive finishing to minimize the quantity of reflected light and to reduce the coupling loss observed for optical coupling and further first and second glass parts 5 and 6 provided with anti-reflection films 7 and 8 respectively are adhered to the corresponding end faces of the optical fibers through adhesive layers 3 and 4, respectively. When this optical fiber type part 10 is practically used, a laser is, for instance, connected to the first optical fiber 1, while a photodiode is, for instance, connected to the second optical fiber 2.
The laser light rays which outgo through the end face of the first optical fiber 1 are converted into light rays approximately parallel to the forward direction during transmitting through the first glass part 5 and the first lens 11 and the light rays are then incident upon the optical isolator 9. The parallel light rays which transmit through and outgo from the optical isolator 9 are condensed into a small-sized beam by the action of the second lens 12, transmit through the second glass part 6 and the majority thereof is incident upon the second optical fiber 2. The light rays condensed by the second lens 12 are incident upon the second glass part 6 without undergoing any reflection due to the presence of the anti-reflection film 8 formed on the second glass part 6 and accordingly, the loss observed on the end face is limited to a very low level on the order of 0.031 to 0.013 dB.
However, the conventional optical fiber type optical part 10 suffers from a variety of drawbacks. For instance, the quantity of transmitting light rays is reduced and the coupling loss in turn increases since air is mixed in the adhesive layers 3 and 4 and the optical path length of the transmitting light rays varies depending on the amount of the adhesive, the angle between the optical axis and the first and second glass parts 5 and 6 adhered to the end faces of the optical fibers 1 and 2 respectively or the environmental temperature and accordingly, the optical characteristics of the optical part are unstable and undergo changes. Moreover, the production of the part 10 takes a long time and becomes expensive since the production thereof requires a process for adhering fine parts such as the first and second glass parts 5 and 6.